This application claims the priority of German Patent Application Serial No. 199 46 057.4, filed Sep. 25, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a tensioner for a power transmission element, such as a chain or belt, of a traction drive for operating, for example, aggregates of an internal combustion engine.
Tensioners of this type automatically implement an even tensioning of the power transmission element. Typically, the mechanical tensioner includes a tension roller which rests against the power transmission element and is rotatably mounted to a swivel arm. The swivel arm is swingably arranged via an axle in a housing which is constrained against executing a rotational movement. A torsion spring tends to urge the tension roller in engagement with the power transmission element by a suitable force. The tensioner further includes a dampening system to dampen high-frequency adjustment movements of the tension roller and swivel arm, introduced by the traction drive into the tensioner in the form of impact loads. The dampening system includes a friction lining and a friction disk, with the friction disk mounted on the swivel arm distal end of the axle and held in abutment against a housing surface.
German Pat. No. DE 44 26 666 describes a tensioner having a friction disk which has one side provided with a circular friction lining that is urged in engagement with a confronting end face of the housing by a torsion spring which applies in addition to a force component upon the swivel arm in circumferential direction also an axial force between the swivel arm and the housing that tends to move apart the swivel arm and the housing. The housing of the tensioner has one end face formed with an axially projecting ring collar for embracing the friction disk on the outside and realizing a flush-mounted installation of the friction disk in the housing. During operation of the tensioner, noise may develop in the area of the adjoining contact surface between the housing and the friction lining and result in an annoying screeching sound, when the friction disk oscillates at high frequency. Due to the structure of this conventional tensioner, a structure-borne noise is transmitted from the friction lining onto friction disk as well as directly onto the housing so that noise radiates via the surface of the tensioner. In addition, the screeching noise compounds and amplifies the noise generated by the internal combustion engine.
It is thus an object of the present invention to provide an improved tensioner, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved tensioner which effectively inhibits screeching noises between rubbing components.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing a housing, a swivel arm having a free end for connection to a rotatable tension roller which rests on the power transmission element, a swivel bearing, including an axle for connection to the swivel arm and a bushing forming part of the housing and supporting the axle, for rotatably supporting the swivel arm in the housing, a spring positioned in a peripheral zone of the swivel bearing between the swivel arm and the housing, a friction disk mounted in fixed rotative engagement to the axle and supported by the housing via a friction lining, and a disk-shaped dampening element positioned between the friction lining and the housing.
Through incorporation of a disk-shaped dampening element between the friction lining and the housing, the friction lining is effectively de-coupled from the housing so that a transmission of noise, generated in the contact area between the friction lining and friction disk, to the housing is effectively inhibited. This measure thus realizes an acoustic decoupling, i.e. a transmission of structure-borne noise is effectively eliminated. Suitably, the disk-shaped dampening element may be securely fixed to the housing and supported by the friction lining.
According to another feature of the present invention, a lid is received in the housing for covering a friction lining distal end face of the friction disk. This can be implemented, for example, by providing the housing with a collar which circumscribes the friction disk and is intended to center and secure the lid in place. The lid is suitably so positioned that a cavity is formed between the friction disk and the lid. Air conduction of noise emanating from the friction disk is hereby effectively eliminated.
Suitably, the friction lining is made of a material having a modulus of elasticity which significantly exceeds a modulus of elasticity of the disk-shaped dampening element. Varying material properties permit a desired high impedance jump between the disk-shaped dampening element and the friction lining. This is advantageous as far as sound absorption capability is concerned.
Sound absorption can be further enhanced by installing a second dampening element in the cavity between the friction disk and the lid. The second dampening element prevents a disadvantageous natural oscillation of the bottom which, without dampening action, acts like a sound board that amplifies the noise.
According to another feature of the present invention, the friction lining may be a separate component which can be installed in a radially centered location between the friction disk and the first dampening element. Through suitable material selection, it is possible to implement that a friction value between the disk-shaped dampening element and the housing significantly exceeds a friction value between the friction disk and the friction lining. This effectively inhibits a movement or displacement between the disk-shaped dampening element and the housing.
The friction lining may be combined with the disk-shaped dampening element to form a structural unit with is mounted rigidly to the housing so as to be constrained from executing a rotational movement. Depending on the material selection, these single components are secured together to the housing, preferably by a material connection, e.g. through ultrasonic sealing or gluing.
According to another feature of the present invention, the disk-shaped dampening element is formed in sandwich construction including two disks made of metallic material and spaced apart at formation of a circular intermediate space, and an elastic filler which fills the intermediate space. A disk-shaped dampening element in sandwich construction represents an optimum solution to provide a wear-resistant, long-lasting structure with an effective dampening action. Moreover, the sandwich construction has the advantage that the disk-shaped dampening element can be pre-fabricated and can be easily suited to neighboring parts, that is the friction lining and the housing. In particular, when the friction lining is a separate component for use in the tensioner, involved here, the disk-shaped dampening element in sandwich construction can be advantageously be used to directly impact the friction value of the rubbing components in the area of the contact surface between the disk-shaped dampening element and the friction lining.
The effectiveness of the disk-like dampening element can be further enhanced through a torsionally stiff configuration and by a dampening action solely in axial direction. The use of such a dampening element is limited to the insulation of structure-borne noise, without influence of friction forces. The disk-shaped dampening element does not exhibit a dampening effect with respect to thrust and torsion and thus does not require an accordingly conformed or newly configured torsion spring and/or greater support surfaces of the spring ends on the housing for influencing the surface pressure.
According to another feature of the present invention, the friction lining has, at least in the area of the contact surface, a hardness which is equal or smaller than a material hardness of the friction disk. In this manner, an even wear of both rubbing components and an increased wear of the friction lining is realized. Thus, a one-sided wear of the friction disk is avoided, and a sufficient strength of the friction disk over the entire service life of the tensioner, utilized for the internal combustion engine, is assured.
Still another feature of the present invention relates to the rubbed-off material generated between the friction disk and the friction lining. To realize an optimum function and in particular to inhibit a noise development that may be experienced after smoothing the surface structure, it is advantageous to continuously remove abraded material released from the contact surface between interacting rubbing components. In this manner, penetration of finest abraded particles into the contact zone at a different location is eliminated. Tests have shown that a contamination of the contact surface by rubbed-off material adversely affects the friction behavior of the rubbing components and oftentimes amplifies a screeching sound of the contacting components in the area of the contact surface.
As the abraded material is predominately of metal, it is proposed according to the invention to provide magnetic strips in the area of the contact zone. A first one of the magnetic strips is disposed in an annular anchoring groove formed in a collar of the housing that circumscribes the friction disk. The anchoring groove covers hereby in concentric relation the contact zone between the friction lining and the friction disk. A second one of the magnetic strips is disposed, radially inwardly offset with respect to the friction lining, directly on the friction disk or on a ring collar which projects out in an axial direction from the friction disk.
The effectiveness of the dampened tensioner according to the present invention can be further enhanced when the second dampening element, disposed in the cavity that is axially bounded by the friction disk and the lid, bridges an axial distance between the lid and the friction disk and is rigidly secured to the friction disk or the lid. Material examples for the second dampening element include silicone or other suitable materials for partially filling the cavity. These materials are easy to incorporate and exhibit a permanent elasticity over the entire temperature range which the tensioner is exposed to.
The cavity may also have installed therein a disk-shaped insulating element which is suitably secured at the outer marginal zone of the friction disk and also bridges an axial distance to the lid. Of course, a disk-shaped insulating element may also be secured to the lid and supported by the friction disk. A suitable material for the insulating element is, for example, elastomer. A disk-shaped insulating element of this type can be reinforced by fabric inlays, e.g. acrylonitrile-butadiene rubber, or nitrile rubber (NBR or HNBR) to exhibit a sufficient strength and to further realize a needed high impedance jump for attaining an effective acoustic decoupling. Other materials suitable for use as insulating element include polyurethane foam (PU-foam) reinforced with fabric plies.